Laser Photodetachment Research Articles

The role of the perturbed plasma potential in the dynamic plasma response to photodetachment

The destruction of negative ions by laser photodetachment suddenly modifies the electron and negative ion densities in a localized region. The subsequent recovery of negative ion density n−(t) may be measured with a second photodetaching laser pulse. These measurements, compared with a ballistic model for n−(t), are the basis of a negative ion temperature diagnostic, which is being developed and employed in H− and D− sources. In this paper the accompanying plasma potential perturbation and its effect on the negative ion response is discussed. The perturbation can introduce an error in negative ion temperature measurements. Late in time, the negative ion density is observed to recover more slowly than indicated by the ballistic theory. An intriguing mechanism is proposed for the late slow recovery of negative ion density, based on a small lingering potential perturbation and a hole in negative ion velocity space. These different theoretical approaches are illustrated with a detailed analysis of an example case from a hydrogen discharge.

Study of weakly-formed negative ions by laser photodetachment and accelerator mass spectrometry

A method of study of weakly-formed negative ions by excitation in laser light and accelerator mass spectrometry is described. Measurement of the photoelectric detachment cross section of the extra electron as a function of the photon energy gives information on the electron affinity. Absolute cross sections for photodetachment of negative ions of La, Ce, Th and U have been measured at the fundamental wavelength (1064 nm) of a Nd: YAG laser. To demonstrate the feasibility of determination of the electron affinity by this method, measurements near the threshold of photodetachment in Te with known electron affinity (1.971 eV) were performed.

Optogalvanic study of a negative ion plasma in a hollow cathode discharge in oxygen

The properties of a negative ion plasma in a hollow cathode discharge have been investigated in oxygen using optogalvanic and probe methods. In the former method, the laser photodetachment technique is applied, and data which support the existence of negative ions are obtained by using CW and pulsed lasers. Appreciable amounts of negative ions are generated between 40 and 150 Pa and at relatively lower discharge currents. It is found that O- or O3- are more dominant at medium currents while O2- is more dominant at lower currents. At high currents, negative ions disappear. The electron energy distribution is depleted in the low-energy region below 1 eV. At higher energies, the energy distribution is almost Maxwellian with an electron temperature of 2.5-3.0 eV. The electron density ranges from 109 to 1010 cm-3 and the ratio of the negative to positive ion density is 0.2-0.6 depending on the pressure and the current.

Plasma diagnostics in negative ion sources

The use of thin probes and laser photodetachment for measuring the negative ion density are described. In the photodetachment method, the work done by detecting the free electrons produced by laser-induced photodetachment is described. Several detection means for the change in electron density produced by laser-induced photodetachment are reviewed: microwave cavity, electrostatic probes and optogalvanic detection. The problems of the choice of the laser and of identification of the negative ions are also discussed. The two-laser photodetachment technique, applied for the measurement of the negative hydrogen and deuterium ion temperature, is described, as are time-resolved measurements of negative ion density.

Carbon cluster imaging at the Weizmann Institute Coulomb explosion system

The structure of small carbon clusters (C n , n = 3–8) has been studied using the Coulomb explosion imaging technique. The experimental system is based on the 14 MV Pelletron accelerator at the Weizmann Institute. Negative molecules are accelerated and photodetached by a laser beam inside the high voltage terminal of the accelerator. The neutral molecules are then electron stripped by passing through a thin Formvar target and the resulting atomic ions are collected by a new 3D multiparticle imaging detector. Nonlinear structures of C 4 and C 5 clusters were observed. New measurements of laser photodetachment thresholds for C n ( n = 3–8) indicate that new compact isomers exist.

The potentialities of H − beam diagnostics by laser photodetachment electrons

The previously proposed method of diagnostics for negative ion beams based on electrons produced from single-photon detachment, is considered. The potentialities of such diagnostics by laser probing of a H − beam are theoretically investigated. Analytical expressions for the power density of a photon target, in which two-photon detachment of ions causes the deterioration of the accuracy in nonperturbative measurements of beam parameters, are obtained. The estimation for the H − beam is made at the Moscow Meson Factory Linac.

Observation of laser excitation of rhombic C4 using the coulomb explosion method

The combination of laser photodetachment of C 4 − and the Coulomb Explosion Imaging method was applied for the investigation of the structure of several C4 isomers and was correlated with their measured electron affinities.

Threshold electron spectra of semiconductor cluster ions

A simple threshold electron analyzer was constructed and laser photodetachment threshold electron spectra of Si− and Si4− were measured.

Laser photodetachment in liquids: C 6F −6 in tetramethylsilane

The photodetachment cross section of C 6F − 6 in liquid tetramethylsilane was measured as a function of the photon energy, using a new two laser photoconductivity technique. The photodetachment threshold was found to be 1.51 eV using a 3 2 power threshold law. Two maxima were observed in the photodetachment cross section at photon energies of 2.58 and 3.15 eV; the cross section value at these two maxima is ≈ 11 × 10 −18 cm 2.

Temporal behavior of the electron and negative ion densities in a pulsed radio-frequency CF4 plasma

Electron and negative ion densities in the afterglow and in the plasma initiation phase of a 13.56-MHz rf discharge in CF4 were measured by using a microwave cavity method and a laser photodetachment technique. Measurements were carried out at low rf powers (≲10 W) and in the pressure range from 100 to 300 mTorr. The electron density in the afterglow showed an enhanced decay rate due to the presence of negative ions. Electrons originating from negative ions through associative collisional detachment with neutral radicals were also detected in the afterglow. Decay curve analysis of the negative ion density gave an ion–ion (presumably CF+3−F−) recombination rate constant of (5±2)×10−13 m3 s−1, and showed that, in the active plasma, the negative ion loss rates by associative detachment and ion–ion recombination are of the same order of magnitude. The behavior of the electron and negative ion densities in the plasma initiation phase indicates that molecules and radicals that slowly accumulate in the plasma do not play a significant role in the production of negative ions.

The structure of small carbon clusters

Abstract New isomers of Cn for n = 4, 5, 6 have been observed using the combination of laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C 4 isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C 5 and C 6 are also presented.

Enhanced volume production of negative ions in the post discharge of a multicusp hydrogen discharge

In this paper we demonstrate a new concept in the production of negative hydrogen ions in a low-pressure multicusp discharge. The discharge voltage is modulated to produce a non-Maxwellian, hot-electron plasma during the current pulse, followed by a cool Maxwellian electron plasma in the post discharge. This procedure, of separating in time the required hot and cold electron plasmas required for volume H− production, is called a temporal filter. The time evolution of the electron energy distribution function is measured using the time-resolved second derivative of a Langmuir probe characteristic. Time-resolved measurements of the negative ion density are made using laser photodetachment. The measurements show that the negative ion density in the center of the source, at a gas pressure of 0.07 Pa, increases by a factor of 2 when the discharge is switched off. At this low pressure the average H− beam current extracted from the source, when operated with a discharge current of 1 A in the pulse modulated mode exceeds the H− beam current from a 5 A continuously operated source. The increase in efficiency of the pulsed source is explained in terms of a two-step H− production mechanism.

The structure of small carbon clusters

New non linear isomers of C n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C4 isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C5 and C6 are also presented.

Observation of doubly excited resonances in the H- ion.

Laser photodetachment on a relativistic H{sup {minus}} beam has revealed several series of resonances in which both electrons are excited. These bound states in the continuum'' appear as dips in the partial production cross sections of various excited states of neutral hydrogen. We present here a detailed account of the experimental observation of the resonances and the results of fits of the measured yields to sets of Fano profiles. A simple analytic formula for the energy-level spacing in this quantum-mechanical three-body system is verified.

Monokinetization of atomic beams by the method of laser photodetachment of electrons

A method is suggested for the generation of atomic beams with a high degree of monokinetization from beams of negative ions accelerated in an electric field up to a threshold moment at which, subject to the Doppler effect, the longitudinal component of the ion velocity becomes sufficient for the photodetachment of an electron from an ion by photons in a laser beam collinear with the ion beam. The resultant neutral atoms continue to move without acceleration and at the same longitudinal velocities equal to the threshold value. An analysis of a number of factors limiting this effect is given below.

Observation of Landau resonances well above threshold in a high-resolution ( Delta f=70 MHz) laser photodetachment study of O-

Magnetic-field effects accompanying ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ photodetachment have been investigated using a high-resolution crossed-beams apparatus. Periodic structures in the photodetachment cross section, due to Landau resonances, have been observed in the previously uninvestigated regime at laser frequencies well above the reaction threshold and at very low magnetic-field strengths. The resonances have a Landau quantum number of about 3065.

Structure and Photochemistry of C4 as Studied by the Coulomb Explosion Imaging Method

AbstractThe Coulomb Explosion Imaging (CEI) method was used to determine the structure and photochemistry of carbon clusters. The cyclic structure of C4 was found using a combination of laser photodetachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than 1 eV lower than the electron affinity of the linear isomer.

The structure of carbon clusters as studied by the coulomb explosion method

The Coulomb explosion imaging (CEI) method has been used to determine the structure and photochemistry of carbon clusters. The cyclic isomer of Cn for 4 ⩽n⩽ 6 was observed, using a combination of laser photo-detachment techniques and the CEI method. The cyclic C4 electron affinity was found to be 2.1 ± 0.1 eV, more than one eV lower than the electron affinity of the linear isomer. Upper limits for the electron affinities of the cyclic C5, and C6 were also established.

Observation of metastable autodetaching Ca-

The predicted 4s4${\mathrm{p}}^{2}$ $^{4}\mathrm{P}$ metastable state of the negative calcium ion has been observed. Measured autodetachment rates yield a lifetime of (2.9\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ s, which is concluded to pertain to the J=(5/2 state. By means of laser photodetachment near the Ca $^{3}\mathrm{S}_{1}$ threshold, the electron affinity of the Ca 4s4p $^{3}\mathrm{P}$ parent state is found to be 0.562\ifmmode\pm\else\textpm\fi{}0.005 eV.

Rotational structure in an excited vibronic band of the dipole-supported state of cyanomethyl anion, CH2CN−

Previous reports of threshold resonances occurring in the photodetachment spectra of molecular anions have provided detailed information about the nature of dipole-supported states and the dynamics of autodetachment from the vibrationless level. In this paper we report the first observation and analysis of rotational band structure in an excited vibrational level of a dipole-supported state. The 1 cm−1 resolution laser photodetachment spectrum of cyanomethyl anion (CH2CN−), the conjugate base of acetonitrile, was recorded in the 12 500–13 700 cm−1 region using ion cyclotron resonance spectrometry. Rotational assignment of the resonances occurring in this region provides evidence for vibrational-to-electronic coupling in the autodetachment process.